Babies are alive and learning in the womb, already forming relationships. (Proof? They come out knowing their mother's voice (and father's if he's present during the pregnancy)). Women are transformed through the process of being pregnant (hormones, brain patterns, and more) and their bodies are perfectly designed to support a little one before and after birth. This entire process forms a deep bond with mother and baby. This bond is the most important first relationship the baby will have and it is foundational to the baby's emotional security later in life (there's a growing body of research). A synthetic womb could NEVER adequately replace a real womb. Furthermore, it's not ethical to even try.
Second, we should examine that pay gap data a bit more. Maybe the problem is that too many women lack well paying jobs at good companies to start with? I know women who took parental leave and came back to a great paying job and positions of leadership. It could be earlier in the funnel where we have the problem.